The projection angle of the conventional automobile headlamp is fixed, therefore being not able to accommodate cars of various heights. Accordingly, some cars are equipped with additional angle adjusting devices, which are shown in FIGS. 1 and 2. The device comprise a bolt 11 attached to a headlamp and connected to the axial hole 121 of a sleeve 12. The free end of the sleeve 12 is provided with a polygonal projection 122 capable of being connected to a teethed ring 131 around a round disk 13. The device further comprises an adjusting bolt 14 vertically crossing the bolt 11 with a gear wheel 15 thereon. The gear wheel 15 is engaged with the round disk 13. The device further includes a fixed mount 16 and a retaining mount 17, respectively housing the bolt 11 and the adjusting bolt 14. Thereby, as the adjusting bolt 14 is twisted, the bolt 11 will move accordingly through the transmission of the gear wheel 15, the round disk 13 and the sleeve 12, by which the projection angle of the headlamp can be adjusted. However, the conventional angle adjusting device needs an extra transmission unit, whereby the adjusting bolt 14 can be moved, and which will take extra space. Meanwhile, there is not a retaining mechanism for restricting the range of shift of the bolt 11, which may lead to problems of over-extension and over-contraction. Therefore, an additional restricting mechanism is needed, which will occupy an extra space. To sum up, the transmission and position-restricting units together will make the conventional angle adjusting device too large.